A hydraulic excavator having a plurality of hydraulic actuators, for example, comprises a hydraulic cylinder for implements, a hydraulic swing motor for an upper swing body, a hydraulic travel motor for a lower travel body and the like as typical actuators. These hydraulic actuators are made to operate by that discharged oil from an actuator pump driven by an engine is supplied to the hydraulic actuators due to control by the hydraulic directional control valves.
As a typical operating means to control the hydraulic directional control valve, a so-called pilot operation is conducted in which pilot oil is supplied to the end portion of a valve spool of the hydraulic directional control valve by a pilot valve to operate the valve spool. This pilot operation is widely utilized for hydraulic excavators, truck cranes and the like, since pilot operation of the hydraulic directional control valve instead of its manual and direct operation reduces operating effort, and enables remote control of the hydraulic directional control valve. Structure of this hydraulic pilot circuit for hydraulic pilot operation will be described below.
(1) The pilot oil is produced and discharged by an engine driven pilot pump with fixed displacement, for example, a gear pump, other than an actuator pump.
(2) The pressure of the discharged pilot oil is set and remained by a relief valve at the pilot pressure necessary for operating the hydraulic directional control valve and the like.
(3) There are provided a plurality of hydraulic directional control valves corresponding to each of the plurality of hydraulic actuators, and a plurality of pilot valves are provided corresponding to each of the plurality of hydraulic directional control valves. When an attachment and the like having the hydraulic actuators are added, the hydraulic directional control valves and the pilot valves for the added hydraulic actuators are additionally mounted.
(4) Regarding the displacement capacity of the pilot pump, the pilot pump with the possible larger displacement capacity is adopted so that the quantity of the pilot oil which is supplied to the pilot valve does not become insufficient, when the plurality of pilot valves are operated at the same time and/or the pilot valves are operated at various speeds. However, it should be noted that when the pilot valves are additionally mounted and increased in number, it causes insufficiency, and therefore changing to a pump with larger discharge may be necessary.